Technology for producing ethanol from a plant biomass material has been actively developed in recent years.
In general, ethanol production from a plant biomass material containing cellulose and/or hemicellulose in a water-insoluble form requires at least a saccharification step and an ethanol fermentation step, or may separately require a delignification step as a pretreatment for the saccharification step.
Examples of a general saccharification step include a sulfuric acid method and an enzyme method. The sulfuric acid method is problematic in high environmental load and corrosion of a reactor. The enzyme method is problematic due to the high cost of cellulase. A biomass resource should generally be pretreated through delignification before being subjected to the enzyme method. An alkaline treatment method and the like are known as delignification steps. The ethanol fermentation step is generally performed using yeast. As described above, a conventional method for producing ethanol from a biomass material requires multiple steps (with multiple stages).
For example, Patent Document 1 discloses a method for producing a substrate for an enzymatic saccharification reaction from a lignocellulose-based biomass raw material through a step involving alkaline treatment; that is, a method for pretreatment of a lignocellulose-based biomass raw material for a saccharification step. In this document, the thus produced substrate is saccharified by an enzyme, and the saccharified product is then further subjected to ethanol fermentation by a microorganism (yeast).
Patent Document 2 describes a saccharification and fermentation system using woody biomass as a raw material. In this system, a step of performing saccharification and fermentation reactions upon the woody biomass using a cellulose-degrading enzyme, a hemicellulose-degrading enzyme, and an alcohol-fermenting microorganism (yeast), and a step of performing a fermentation reaction using a microorganism (yeast) capable of separating a pentose substance remaining in the reaction product, and alcohol-fermenting pentose are implemented.
Patent Document 3 describes a method by which enzymatic saccharification and alcoholic fermentation are performed in the same fermenter. However, even this method requires the combined use of a microorganism such as filamentous fungi capable of producing a saccharifying enzyme and yeast capable of performing alcoholic fermentation.
As described above, conventional technology for producing ethanol from a plant biomass material requires multiple steps (with multiple stages), and thus is not satisfactory in terms of energy consumption and cost. Technology for implementing a delignification step, a saccharification step, and an ethanol fermentation step with the use of a single means has been required.
Plant biomass materials such as rice straws, bamboos, and hardwood lumber contain hemicellulose, which contains a pentose (e.g., xylose) as a constituent unit. However, general ethanol-fermenting microorganisms are not capable of assimilating a pentose to generate ethanol. Therefore, the production of ethanol using a general ethanol-fermenting microorganism from a saccharified product of hemicellulose is problematic, in that the pentose cannot be used and the ethanol yield is lowered. As described in Patent Document 2, the combined use of a microorganism capable of assimilating a hexose such as glucose, so as to ferment and generate ethanol and a microorganism capable of assimilating a pentose such as xylose, so as to ferment and generate ethanol is problematic in terms of cost and the resulting complicated reaction system.
Patent Document 4 discloses, a method for producing alcohol from a carbon source by alcoholic fermentation using Trametes suaveolens as technology that enables implementation of a delignification step, a saccharification step, and an ethanol fermentation step with a single means. However, in Patent Document 4, Trametes suaveolens was confirmed to be able to assimilate hexose, however, whether or not Trametes suaveolens is capable of simultaneously assimilating pentose and hexose was not confirmed. Specifically, technology, by which a delignification step, a saccharification step, and an ethanol fermentation step can be implemented using a single means, which would allow both hexose and pentose to be ethanol fermented as carbon sources, has not yet been established.
Trametes suaveolens used in Patent Document 4 is a type of white-rot fungi. White-rot fungi are known to have the ability to produce lignin peroxidase, manganese peroxidase, laccase, and the like and to degrade wood-derived lignin (Patent Document 5). White-rot fungi are also known to have the ability to saccharify cellulose. The Phlebia sp. MG-60 strain belonging to the genus Phlebia has been isolated as a type of white-rot fungi, which is capable of degrading lignin under hypertonic conditions (Patent Document 6). Whether or not white-rot fungi belonging to the genus Phlebia has the ability to ferment alcohol has conventionally remained unexamined.